Index: Lib/test/test_telnetlib.py
===================================================================
--- Lib/test/test_telnetlib.py	(revision 74347)
+++ Lib/test/test_telnetlib.py	(working copy)
@@ -305,6 +305,43 @@
             sb_data = self.sb_getter()
             self.sb_seen += sb_data
 
+class SocketSendall(socket.socket):
+    _raw_sent = b''
+    def sendall(self, data):
+        self._raw_sent += data
+
+class TelnetSockSendall(telnetlib.Telnet):
+    def open(self, host, port=0, timeout=socket._GLOBAL_DEFAULT_TIMEOUT):
+        ''' a near-exact copy of Telnet.open '''
+        self.eof = 0
+        if not port:
+            port = tl.TELNET_PORT
+        self.host = host
+        self.port = port
+        self.timeout = timeout
+        self.sock = SocketSendall()
+
+class WriteTests(TestCase):
+    '''The only thing that write does is replace each tl.IAC for
+    tl.IAC+tl.IAC'''
+    def _test_write(self, data):
+        self.telnet.write(data)
+        after_write = self.telnet.sock._raw_sent
+        self.assertEqual(data.replace(tl.IAC,tl.IAC+tl.IAC),
+                         after_write)
+
+    def test_write(self):
+        self.telnet = TelnetSockSendall('some.host.net')
+        data_sample = [b'data sample without IAC',
+                       b'data sample with' + tl.IAC + b' one IAC',
+                       b'a few' + tl.IAC + tl.IAC + b' iacs' + tl.IAC,
+                       tl.IAC,
+                       b'']
+        for d in data_sample:
+            self.telnet.open('some.host.net')
+            self._test_write(d)
+            self.telnet.close()
+
 tl = telnetlib
 
 class TelnetDebuglevel(tl.Telnet):
@@ -410,7 +447,7 @@
         return
 
 def test_main(verbose=None):
-    support.run_unittest(GeneralTests, ReadTests, OptionTests)
+    support.run_unittest(GeneralTests, ReadTests, WriteTests, OptionTests)
 
 if __name__ == '__main__':
     test_main()